Field of the Invention
The present invention relates to a fixing device and an image forming apparatus.
Description of the Related Art
A conventional image forming apparatus includes an image forming unit that forms an image by applying toner onto a paper sheet, and a fixing device that fixes the toner to the paper sheet by heating and pressing the paper sheet having the toner applied thereto. The fixing device may be a device that heats and presses a paper sheet with a nip portion between a pair of rollers including a fixing roller having a heater installed therein, or a device that heats and presses a paper sheet via a fixing belt stretched around the fixing roller.
A halogen lamp heater or the like is normally used as a heater in a heating roller. Switch on/off control is normally performed to control a halogen lamp heater. However, to perform energization in a more specific manner, Duty control depending on an on/off time ratio is performed through phase control.
As examples of such Duty control depending on a time ratio, the following control operations have been suggested: a control operation in which a halogen lamp heater is switched on in accordance with the color temperature of the halogen lamp heater, except for the time Duty during which a chemical attack is likely to occur (see JP 2012-53148 A, for example); and a control operation in which a check is made to determine whether a chemical attack has occurred by calculating deviation of the halogen gas density in the tube from the color temperature, and a halogen lamp heater is switched on to cancel the deviation of the halogen gas density (see JP 2012-63644, for example). However, Duty control depending on a time ratio involves minute segmentation of alternating current. As a result, terminal noise or harmonic noise is generated, and the antinoise circuit and components become complicated.
To counter this problem, half-wave control can be performed as another method of controlling a halogen lamp heater. In half-wave control, energization or de-energization is performed for each half-wave of an AC waveform, and the effective voltage to be applied to the halogen lamp heater differs from the rated voltage.
However, if the above mentioned half-wave control is performed on a conventional halogen lamp heater, a chemical attack might occur, since the effective voltage differs from the rated voltage.
In this halogen lamp heater, tungsten gasified from the tungsten filament due to a temperature rise binds to the halogen gas contained in the halogen lamp heater, to generate tungsten halide. The tungsten halide is thermally decomposed by the filament, and the tungsten is deposited on the filament. That is, a halogen cycle occurs. The above mentioned chemical attack is a phenomenon that occurs in a situation where tungsten is not gasified from a low-temperature filament. More specifically, the halogen gas reacts directly with the tungsten of the filament, to form a tungsten halide and gasify. However, due to the low filament temperature, the tungsten halide cannot be thermally decomposed, and the filament becomes gradually thinner, without any tungsten deposited on the filament.